1. Field of the Invention
The present invention relates to a novel sustained release drug delivery device, and more particularly to a multilayered drug delivery device.
2. Brief Description of the Related Art
Over the years, various drugs have been developed to assist in the treatment of a wide variety of ailments and diseases. However, in many instances such drugs are not capable of being administered either orally or intravenously without the risk of various deleterious side effects.
For example, intravenous ganciclovir (GCV) is effective in the treatment of cytomegalovirus (CMV) retinitis in AIDS patients, but bone marrow toxicity limits its usefulness. The incidence of neutropenia (absolute neutrophil count<1000) during intravenous GCV therapy ranges from 30 to 50%. Continuous maintenance GCV therapy is necessary to prevent progression or recrudescence of the disease, but despite maintenance therapy, 30 to 50% of patients experience a relapse during treatment. Other problems associated with systemic GCV administration include the risk of sepsis related to permanent indwelling catheters and the inability to receive concurrent therapy with zidovudine (AZT) which has been shown to prolong life and improve the immune function in AIDS patients.
Intravitreal GCV injections of 200 to 400 μg administered once or twice weekly have resulted in temporary remission of CMV retinitis in AIDS patients. Intravitreal GCV injections may provide a higher intraocular drug concentration than systemic therapy and reduce the incidence of neutropenia. Current treatment of CMV retinitis in AIDS patients is clearly suboptimal. Ganciclovir is virustatic and thus disease inhibition requires maintenance drug administration.
Due to the risks that certain drugs impose, researchers have developed systems for administering such drugs to aid in the treatment of these ailments and diseases. Many of these systems provide a release rate which reduces the occurrence of detrimental side effects.
With conventional dosing (tablets, injections, etc.), the concentration of drug in a given area of the body increases through an ineffective concentration to some concentration that is effective. Frequently the concentration may actually reach some toxic threshold. After a relatively short period, however, the drug concentration decreases as drug is either metabolized in the body or is eliminated. Frequently, drug levels decrease so low that therapeutic levels are no longer maintained. A second dose is then given and the cycle is repeated. The goal of sustained release systems is to maintain drug levels within the therapeutic range and ideally a constant level.
In order to achieve constant levels, drugs should be released from a delivery system at a rate that does not change with time (so called zero-order release). In many systems, however, the release rate is proportional to time (i.e., “first order”) or the square root of time (or Fickian).
Linear release is achievable with some types of reservoir systems, such as tubes, fibers laminates, or microspheres. In these systems, a drug reservoir is coated in a rate controlling membrane. Drug diffusion across the membrane is rate limiting and is constant (zero order) as long as the membrane's permeability does not change and as long as the concentration of drug in the reservoir is constant (i.e. as long as there is an excess of drug in the reservoir).
In matrix systems, drug is dispersed throughout a matrix and is released as it dissolves and diffuses through the matrix. A drug is released from the outer surface of the matrix first, this layer becomes depleted, and drug that is released from further within the core of the device must then diffuse through the depleted matrix. The net result is that the release rate slows down and Fickian release is common. With matrix systems, zero-order release is very difficult to achieve. The same principles apply to release from gels.
In some bioerodible systems, diffusion through the matrix is designed to be extremely slow and drugs are intended to be released as the system is degraded. It has been generally found to be extremely difficult to use this approach to achieve a zero-order release, as most polymers do not undergo zero-order degradation. “S” type kinetics are more common. A general discussion of drug delivery control systems is provided in Controlled Drug Delivery Systems (Part I), Xue Shen Wu, Ph.D. pp. 32, 33, 44-46, 63, 66, and 67 (Technomic Publishing Co, Inc., 1996), the entire contents of which are incorporated herein by reference.
One such delivery device is an orally administered pill or capsule which contains a drug encapsulated within various layers of a composition that dissolves over a period of time in the digestive tract, thereby allowing a gradual or slow release of the drug into the system.
Another type of device for controlling the administration of such drugs is produced by coating a drug with a polymeric material permeable to the passage of the drug to obtain the desired effect. Such devices are particularly suitable for treating a patient at a specific local area without having to expose the patient's entire body to the drug. This is advantageous because any possible side effects of the drug could be minimized.
Such systems are particularly suitable for treating ailments affecting the eye. Advances for administering a drug to the external surface of the eye are disclosed in U.S. Pat. No. 4,014,335, to Arnold. Arnold describes various ocular inserts that act as a deposit or drug reservoir for slowly releasing a drug into the tear film for prolonged periods of time. These inserts are fabricated of a flexible polymeric material that is biologically inert, non-allergenic, and insoluble in tear fluid. To initiate the therapeutic programs of these devices, the ocular inserts are placed in the cul-de-sac between the sclera of the eyeball and the eyelid for administering the drug to the eye.
Devices formed of polymeric materials that are insoluble in tear fluid retain their shape and integrity during the course of the needed therapy to serve as a drug reservoir for continuously administering a drug to the eye and the surrounding tissues at a rate that is not effected by dissolution or erosion of the polymeric material. Upon termination of the desired therapeutic program, the device is removed from the cul-de-sac.
Another type of device used for sustained release of a drug to the external surface of the eye, described in U.S. Pat. No. 3,416,530, is manufactured with a plurality of capillary openings that communicate between the exterior of the device and the interior chamber, which generally defined from a polymeric membrane. While these capillary openings in this construction are effective for releasing certain drugs to the eye, they add considerable complexity to the manufacture of the device because it is difficult to control the size of these openings in large scale manufacturing using various polymers.
Another device, described in U.S. Pat. No. 3,618,604, does not involve such capillary openings, but instead provides for the release of the drug by diffusion through a polymeric membrane. The device, in a preferred embodiment, as disclosed in that patent, comprises a sealed container having the drug in an interior chamber. Nonetheless, as described in U.S. Pat. No. 4,014,335, certain problems have been identified with such devices such as the difficult task of sealing the margins of the membrane to form the container. In addition, stresses and strains introduced into the membrane walls from deformation during manufacturing of those devices may cause the reservoir to rupture and leak.
Another such device, described in U.S. Pat. No. 4,014,335, comprises a three-layered laminate having a pair of separate and discrete first and third walls formed of a material insoluble in tear fluid with one of the walls formed of a drug release material permeable to the passage of drug and the other wall formed of a material impermeable to the passage of the drug.
The above described systems and devices are intended to provide sustained release of drugs effective in treating patients at a desired local or systemic level for obtaining certain physiological or pharmacological effects. However, there are many disadvantages associated with their use including the fact that it is often times difficult to obtain the desired release rate of the drug. The need for a better release system is especially significant in the treatment of CMV retinitis.
Prior to the development of the present invention, there was developed a novel sustained release delivery device which ameliorated many of the aforementioned problems associated with drug delivery. The device, which is disclosed in U.S. Pat. No. 5,378,475 (incorporated herein by reference in its entirety), included a first coating essentially impermeable to the passage of the effective agent and a second coating permeable to the passage of the effective agent. In the device, the first coating covered at least a portion of the inner core; however, at least a small portion of the inner core is not coated with the first coating layer. The second coating layer essentially completely covers the first coating layer and the uncoated portion of the inner core. The portion of the inner core which is not coated with the second coating layer allows passage of the agent into the second coating layer thus allowing controlled release.
While the devices described in U.S. Pat. No. 5,378,475 solve many of the aforementioned problems pertaining to drug delivery, the devices and the method of making the devices are not without problems. In particular, polymers suitable for coating the inner core are frequently relatively soft and technical difficulties can arise in production of uniform films. This is especially true when attempting to coat non-spherical bodies with edges (such as a cylindrical shape). In such case, relatively thick films must be applied to achieve uninterrupted and uniform coatings, which adds significant bulk to the device. Thus, the devices tend to be larger than necessary as a result of the thickness needed to seal the ends of the inner core.
The aforementioned U.S. Pat. No. 5,902,598 presents solutions to the problems of manufacturing devices which are sized to be effective drug administration devices in anatomical locations where device volume plays a limiting role in the design of the device. While effective in delivering drugs in situ, some manufacturing difficulties have limited scaled up manufacturing of these devices. For example, the impermeable inner coating layer of the devices of the aforementioned application, which immediately surrounds the drug reservoir, is typically formed of a material the thickness of which results in a layer which is not capable of supporting its own weight.
While beneficial from the standpoint of reducing the overall size of the device, and while still sealing the drug reservoir, goals well-addressed in the aforementioned patent, the relative flaccidity of this inner layer makes it difficult to load the device's reservoir with a drug solution, drug slurry, or drug suspension. Because this inner layer is essentially structurally incapable of maintaining its shape without significant collapse, i.e., does not have the dimensional stability or structural ability to accept a drug core inserted therein without changing shape, a relatively solid drug or drug-containing mixture must be used in order to manufacture the device. Loading a drug slurry onto this inner layer during manufacture which does not hold its own shape results in the combination of the drug slurry and inner layer being extremely difficult to handle without damaging it, because the inner layer collapses and the drug-containing mixture flows out.
The problem of device size is extremely important in the design of devices for implantation into limited anatomical spaces such as the eye. Larger devices require more complex surgery to both implant and remove. The increased complexity may result in complications, longer healing or recovery periods, and potential side effects (e.g., increased chance of astigmatism). Furthermore, the extra polymer required to achieve a uniform coating reduces the potential volume of the implant and hence limits the amount of drug that can be delivered, potentially limiting both efficacy and duration.
As a result of all of the above, there remains a need in the art for improving the design and the method of preparing devices which provide a sustained release of a drug to a patient to obtain a desired local or systemic physiological or pharmacological effect especially for ocular use.